1. Technical Field
The disclosed embodiments relate to Low Noise Amplifiers (LNAs), and more particularly to common gate LNAs.
2. Background Information
LNAs are used in many applications, including use in cellular telephone receivers. A signal received onto an antenna of such a receiver is typically weak and requires amplification for subsequent stages of the cellular telephone operation. An LNA is typically used to amplify such a signal. In such an application, the LNA should introduce as little noise as possible into the system. Noise generated by a poor LNA may be amplified during subsequent stages and could result in poor phone reception. In addition to having good noise performance, many LNAs today are to be operable over a wide frequency range. Two typical architectures are usually utilized to realize these performance objectives: common source LNAs and common gate LNAs. However, problems exist with both LNA architectures as described below.
FIG. 1 (Prior Art) is a simplified block diagram of one way of accomplishing wideband LNA operation. Rather than employing one wideband LNA, multiple narrow band common source LNAs are employed where each narrow band LNA operates over a different part of the wide frequency range to be served. Each LNA may operate in a different narrow frequency band that is usually less than 100 MHz wide. Each of the narrow band LNAs requires its own filter and matching components. In some cases, ten frequency bands of operation are required so ten LNAs are required, and ten filters are required, and ten sets of matching components are required. Providing all this hardware is costly and large and consumes a lot of power.
FIGS. 2, 3 and 4 (Prior Art) are circuit diagrams of wideband non-tunable LNAs. FIG. 2 is a circuit diagram of a differential non-tunable common gate LNA. LNA 1 is said to be “non-tunable” because its input impedance can not be controlled and its input impedance may change as a function of the frequency of the signal being amplified. Since LNA 1 cannot be tuned, LNA 1 may exhibit poor noise performance in some applications. LNA 1 also utilizes off chip inductors which are costly and use board area. Additionally, positive feedback transistor 2 generates noise and decreases the noise performance of the LNA. FIG. 3 is a circuit diagram of a single input, differential output common gate LNA. LNA 3 is also non-tunable and has poor noise characteristics in some operating conditions. FIG. 4 is a diagram of a first stage of a differential non-tunable common gate LNA. LNA 4 is also non-tunable and has noise problems in some operating conditions.
FIG. 5 (Prior Art) is a circuit diagram of a wideband tunable LNA referred to as a Positive Feedback Common Gate LNA (PFCGLNA). LNA 5 can be tuned so that its input impedance matches the impedance of the source driving the LNA, but LNA 5 has instability and performance problems. LNA 5 has P-channel positive feedback transistors and N-channel input transistors. The positive feedback and input transistors should be matched in order for LNA 5 to be stable. Label M1 in FIG. 5 identifies one of the input transistors. Label M2 in FIG. 5 identifies a positive feedback transistor that should be matched to input transistor M1. Maintaining this matched condition despite process variations in the semiconductor manufacturing processes used to fabricate the PFCGLNA is difficult. In addition to instability problems, LNA 5 also suffers noise performance problems. Under certain operating conditions, noise generated by the LNA's positive feedback circuitry is amplified. A wideband tunable common gate LNA with improved stability and noise characteristics is desired.